1. Field of the Invention
This invention relates to integrated circuit manufacturing and more particularly, to forming an interconnect using tungsten as a hard mask.
2. Description of the Related Art
Current integrated circuit manufacturing processes typically use multiple levels of some form of metal interconnect to provide interconnections between various circuits on the integrated circuit. For instance, current manufacturing processes are known to use five levels of metal interconnects. The etch process to form the interconnect requires that precisely defined regions of metal be exposed to etchants in order to selectively remove material from those regions. The patterns that define the regions are typically created using lithographic processes. For instance, a layer of photoresist is spin-coated onto a substrate, and the resist layer is selectively exposed to a form of radiation, such as ultraviolet light, electrons, or x-rays.
An exposure tool and mask, or data tape in electron beam lithography, are used to effect the desired selective exposure. Openings in the resist corresponding to the selective exposure are formed in a subsequent development step. Next, an etch is applied, and the regions of the substrate exposed by the openings are removed. The etching provides a desired pattern in the metal layer to form the final usable interconnect.
Current lithographic processes are approaching the limits of the ability of to resolve minimum features that are adequate for a manufacturing process. Both line or space resolution and depth of focus are related to the numerical aperture and wavelength by the following expressions: EQU Resolution=k1.lambda./NA EQU Depth of Focus=k2.lambda./NA.sup.2
where k1 and k2 are resist coefficients, .lambda. is the stepper wavelength and NA is the numerical aperture of the stepper. For a typical I line process, .lambda. is 365 mm, NA is 0.5 and k1=0.65 and k2=1.0. Given these sets of conditions it is easily shown that limits for resolution and depth of focus values are already being stretched by the established 0.35 micron technology and emerging 0.25 micron technology.
The process limitations are further compounded by poor selectivity between metal and photoresist during a typical metal etch process. The photoresist performs two primary functions: precise pattern formation, and protection of the substrate during etch. Photoresist resolution refers to the ability of the photoresist to accurately match the pattern of the lithographic exposure equipment. Photoresist resolution is a key parameter that depends on several factors including the contrast, swelling, and thickness of the photoresist, as well as proximity effects (e.g., an isolated opening). As the photoresist becomes thicker, the openings near the bottom surface tend to narrow. As a result, thicker photoresist tends to limit the resolution. As the thickness of metal films are increased, thicker and thicker photoresist must be used in order to provide enough process margin during the etch. That is, in order to prevent excessive metal erosion during an etch, sufficient photoresist must be left over protected metal areas during the metal etch portion of the process. However, as resists become thicker, it becomes harder to resolve smaller features with an acceptable depth of focus.
Therefore, generally speaking, the resolution is improved by thinning the photoresist. That is, thinner resist allows extending standard exposure tools to finer geometries. However, the photoresist must remain thick enough to avoid metal being eroded by the etchant. For instance, for submicron patterns, the photoresist might be approximately 2 to 3 times as thick as an underlying metal layer in order to adequately protect the metal layer from an etchant that is particularly harsh to the photoresist. Unfortunately, at this thickness, the resolution of the photoresist may produce significant deviations between the desired pattern and the actual pattern transferred to the substrate. Accordingly, there exists a need for an efficient method of patterning metal layers forming interconnects using a thin layer of photoresist while adequately protecting the metal film from the etch.